The conventional retaining mechanism of the vertebral fixation rod is generally composed of screws for fastening the vertebral fixation elements, such as bone screws, bone hooks, etc., with the vertebral fixation rod. The case in point is the U.S. Pat. No. 5,257,993 disclosing an invention in which the set screw 60 is used to fasten the rod 48 to a receiving slot of the vertebral fixation rod, which is formed by the U-shaped end opening 52 of the hook portion 22 and the transverse opening 90 of the end cap 24. A similar invention is disclosed in the U.S. Pat. No. 4,887,596 in which the fixation rod is fastened by means of adjustment screws 22 and 23. Such prior art methods of fastening an object with a fixation rod by means of the adjustment screws as described above are inherently defective in design in that the adjustment screws are vulnerable to becoming loosened by the vertebral movement, and that the effect of fastening the object with the fixation rod is seriously undermined.